This invention relates to methods and apparatus for liquid precursor vaporization for thin film deposition and semiconductor device fabrication. The precursor liquid is vaporized in a precise and controlled manner to generate a high purity gas/vapor mixture that is substantially free of particulate contaminants. The gas/vapor mixture is then introduced into a chamber for film deposition and semiconductor device fabrication.
Thin film formation by chemical vapor deposition (CVD) is a well-known process in semiconductor device fabrication. In conventional CVD, a precursor vapor is introduced into a chamber in which one or more semiconductor wafers are held at a suitable temperature and pressure to form a thin film on the wafer surface. Insulating, conducting and semi-conducting thin films can be formed by the CVD process using suitable precursor chemicals. If the precursor is a liquid at room temperature, the liquid must be vaporized to form a vapor for film deposition. The process is often referred to as metal organic CVD, or MOCVD, if the precursor liquid is a metal organic compound. Apparatus for liquid precursor vaporization plays an important role in CVD and MOCVD applications. It must be designed properly and be capable of generating vapor with repeatability and accuracy to achieve uniform, high quality thin films for commercial device fabrication in semiconductor and related industries.
When the desired film thickness is small and approaches a few nanometers in overall thickness, an Atomic Layer Deposition (ALD) process can be used. In ALD two complementary vapor pairs are used. One, such as ammonia, is first chemisorbed onto the wafer surface to form a monolayer of molecules of the first vapor. A second vapor is then introduced into the chamber to react with the first chemisorbed vapor layer to form a single monolayer of the desired film. The process is repeated as many times as is necessary in order to form multiple atomic film layers with the desired overall thickness. The ALD process produces film with good step coverage and excellent conformity to the topography and underlying surface structure on the wafer. The film thickness can also be precisely controlled. For these reasons, ALD is finding increasing use in advanced semiconductor device fabrication involving small geometrical dimensions.
Vapor generation in a controlled manner is possible by direct liquid injection. Direct liquid injection is accomplished through a direct liquid vaporizer (DLI vaporizer), in which liquid is injected into a heated chamber for vaporization. The method is generally limited to vapor generation at rates that are higher than a few milligrams per second. When the desired vapor generation rate is low, it becomes increasingly more difficult to control the small amount of liquid that needs to be injected. Alternative methods must then be used.
A commonly used alternative is the bubbler. In a conventional bubbler, a carrier gas is bubbled through a heated precursor liquid to saturate the gas with vapor. The gas/vapor mixture then flows into the CVD or ALD chamber by opening and closing valves. Such prior art bubblers are shown and described in U.S. Pat. Nos. 5,288,325 and 6,579,372.
A prior art ALD deposition system is described by Hausmann et al. (Atomic Layer Deposition of Hafnium and Zirconium oxides using metal amide precursors), Chem. Meter. 14, 43-50-4358, 2002). An external volume is used to control the amount of vapor to be delivered to the deposition chamber. By opening and closing the on-off valves connected to the external volume, the external volume is first filled with vapor from a heated vaporization chamber and then emptied into the deposition chamber. Dielectric thin films such as metal oxides and nitrides including SiO2, HfO2, Zro2, WO3, and WN have been deposited by this method using ALD (Becker et al., Diffusion barrier properties of tungsten nitride films grown by atomic layer deposition from bis(tertbutylimido) bid(dimethylamido)tungsten and ammonia, applied physics letters, Vol. 82, No. 14, 7 Apr. 2003; Hausmann et al. Surface morphology and crystallinity control in the atomic later deposition (ALD) of hafnium and zirconium oxide thin films, Journal of Crystay Growth, 249, pgs. 251-261, 2003).
FIG. 1 illustrates the bubble formation process in a conventional prior art bubbler. Liquid is placed in a metal container, 50, which is usually heated. A carrier gas is introduced into the bubbler through inlet tube 52. As the gas leaves the bottom of tube 52, it forms a stream of bubbles, 54. As each bubble rises through the liquid, the surrounding liquid pressure decreases, causing the bubble volume to expand causing the bubble to rise to the surface quickly. As the bubble bursts through the liquid surface, small droplets, 56, are formed, which are then carried by the gas/vapor mixture through the outlet tube 58.
One disadvantage of the prior art bubbler is that the precursor liquid must be placed in a heated vessel for a prolonged period. Prolonged thermal contact between the liquid and the hot vessel walls can cause the precursor liquid to thermally decompose to form undesirable by-products. Another disadvantage is that with increased gas flow, the gas bubbles would rise more quickly to the liquid surface thereby reducing the residence time of the bubbles in the liquid, thereby causing the gas to become less saturated with vapor. With the bubbler, the vapor generation rate is often unknown or uncontrolled. In addition, as the bubbles burst at the liquid surface, liquid is atomized to form droplets that are entrained by the carrier gas to deposit in the downstream components. Such components as valves, fittings, tubing connection, as well as the deposition chamber are often coated with precursor droplets that have impacted on the heated metal surface and subsequently under thermal decomposition to form a non-volatile residue coating the surface. Over time, the system components would become contaminated. The thin film deposition tool itself must then be shut down for maintenance and cleaning, resulting in the loss of productivity of the tool.
A porous metal wall with interstitial spaces extends from the liquid reservoir for containing liquid from the reservoir.